Single Fiber Passive Optical Network Wavelength Division Multiplex Overlay

ABSTRACT

A downstream baseline optical signal generated by a baseline optical link is combined with a second optical signal generated by an additional optical link to generate a broadband optical signal. A fiber optic network splits the broadband optical signal to a plurality of fiber drops. An optical network termination unit receives the broadband optical signal from the fiber drop and isolates the downstream baseline optical signal and the second downstream optical signal from the broadband optical signal. An upstream baseline optical signal may also be transmitted through the fiber drop by the optical termination unit. The optical termination unit does not effect baseline optical service to any other optical termination unit in the fiber optic network.

RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 10/199,566 that has issued into U.S. Pat. No. 7,254,330, whichclaimed priority from and is related to U.S. Provisional Application No.60/306,907. These prior applications, including the entire writtendescription, claims, and drawing figures, are hereby incorporated intothe present application by reference.

TECHNICAL FIELD

The present invention relates generally to the field of broadbandmulti-media communication systems. More specifically a single fiberpassive optical network wavelength division multiplex overlay isprovided that enables a baseline single fiber passive optical network tobe upgraded on a subscriber by subscriber basis with a multiplicity ofdifferent optical communication links operating at different opticalwavelengths from the baseline link.

BACKGROUND

Prior to the explosive growth in the public's demand for data services,such as dial-up Internet access, the local loop access networktransported mostly voice information. This present access networktypically includes numerous twisted-pair wire connections between theplurality of user locations and a central office switch (or terminal).These connections can be multiplexed in order to more efficientlytransport voice calls to and from the central office. The present accessnetwork for the local loop is designed primarily to carry these voicesignals, i.e., it is a voice-centric network.

Today, data traffic carried across telephone networks is growingexponentially, and by many measures may have already surpassedtraditional voice traffic, due in large measure to the explosive growthof dial-up data connections. The basic problem with transporting datatraffic over this voice-centric network, and in particular the localloop access part of the network, is that it is optimized for voicetraffic, not data. The voice-centric structure of the access networklimits the ability to receive and transmit high-speed data signals alongwith traditional quality voice signals. Simply put, the access part ofthe network is not well matched to the type of information it is nowprimarily transporting. As users demand higher and higher datatransmission capabilities, the inefficiencies of the present accessnetwork will cause user demand to shift to other mediums of transportfor fulfillment, such as satellite transmission, cable distribution,wireless services, etc.

An alternative present local access network that is available in someareas is a digital loop carrier (“DLC”) system. DLC systems utilizefiber-optic distribution links and remote multiplexing devices todeliver voice and data signals to and from the local users. An early DLCsystem is described in U.S. Pat. No. 5,046,067 titled “DigitalTransmission System” (“the '067 patent”). The '067 patent describes aDigital Loop Carrier (DLC) system. In a typical DLC system, a fiberoptic cable is routed from the central office terminal (COT) to a hostdigital terminal (HDT) located within a particular neighborhood.Telephone lines from subscriber homes are then routed to circuitrywithin the HDT, where the telephone voice signals are converted intodigital pulse-code modulated (PCM) signals, multiplexed together using atime-slot interchanger (TSI), converted into an equivalent opticalsignal, and then routed over the fiber optic cable to the centraloffice. Likewise, telephony signals from the central office aremultiplexed together, converted into an optical signal for transportover the fiber to the HDT, converted into corresponding electricalsignals at the HDT, demultiplexed and routed to the appropriatesubscriber telephone line twisted-pair connection.

Some DLC systems have been expanded to provide so-calledFiber-to-the-Curb (FTTC) systems. In these systems, the fiber opticcable is pushed deeper into the access network by routing fiber from theHDT to a plurality of Optical Network Units (ONUs) that are typicallylocated within 500 feet of a subscriber's location. Multi-media voice,data, and even video from the central office location is transmitted tothe HDT. From the HDT, these signals are transported over the fibers tothe ONUs, where complex circuitry inside the ONUs demultiplexes the datastreams and routes the voice, data and video information to theappropriate subscriber.

SUMMARY

A single fiber passive optical network (PON) wavelength divisionmultiplex (WDM) overlay includes a central office, a fiber opticnetwork, and a plurality of optical network termination (ONT) units. Thecentral office has a baseline optical link that receives a baselinecommunication signal and converts the baseline communication signal intoa downstream baseline optical signal within a first optical bandwidth,and has an additional optical link that receives a second type ofcommunication signal and converts the second type of communicationsignal into a second downstream optical signal within a second opticalbandwidth. The downstream baseline optical signal generated by thebaseline optical link is combined with the second optical signalgenerated by the additional optical link to generate a broadband opticalsignal. The fiber optic network is coupled to the central office andreceives the broadband optical signal on at least one optic fiber. Thefiber optic network splits the broadband optical signal to a pluralityof fiber drops. The optical network termination (ONT) units are eachcoupled to a fiber drop. At least one of the ONT units is a baseline ONTunit that receives the broadband optical signal from the fiber drop andsplits the downstream baseline optical signal from the broadband opticalsignal, and at least one other of the ONT units is an upgraded ONT unitthat receives the broadband optical signal from the fiber drop andsplits the downstream baseline optical signal and the second downstreamoptical signals from the broadband optical signal. In addition, theinstallation of the upgraded ONT unit to the PON does not effectbaseline optical service to any other ONT unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary single fiber passive opticalnetwork wavelength division multiplex overlay that includes an enhancedband broadcast sub-carrier modulated (SCM) signal upgrade;

FIG. 2 is a graph illustrating exemplary bandwidths for thecommunication signals transmitted over the single fiber passive opticalnetwork shown in FIG. 1;

FIG. 3 is a block diagram of an exemplary single fiber passive opticalnetwork wavelength division multiplex overlay that includes a coursewavelength division multiplexing (CWDM) T-Band upgrade;

FIG. 4 is a graph illustrating exemplary bandwidths for thecommunication signals transmitted over the single fiber passive opticalnetwork shown in FIG. 3;

FIG. 5 is a block diagram of an exemplary single fiber passive opticalnetwork wavelength division multiplex overlay that includes a densewavelength division multiplexing (DWDM) L-Band upgrade; and

FIG. 6 is a graph illustrating exemplary bandwidths for thecommunication signals transmitted over the single fiber passive opticalnetwork shown in FIG. 5.

DETAILED DESCRIPTION

I. Enhanced Band Broadcast Sub-Carrier Modulated (SCM) Upgrade

Referring now to the drawing figures, FIG. 1 is a block diagram of anexemplary single fiber passive optical network wavelength divisionmultiplex overlay 10 that includes an enhanced band broadcastsub-carrier modulated (SCM) signal upgrade. The system 10 includes aplurality of baseline optical network termination (ONT) units 12, atleast one sub-carrier modulated (SCM) upgraded ONT unit 14, a fiberoptic network 18-23, and a central office 16. The system 10 ispreferably a passive optical network, such as a fiber to the home (FTTH)or fiber to the curb (FTTC), or fiber to the business (FTTB), that maybe upgraded to include a SCM signal, such as a CATV television signal ora DBS signal, on a subscriber by subscriber basis without affectingnon-upgraded subscribers.

Multimedia communication signals, such as plain-old-telephone signals(POTS), data network signals, CATV signals and DBS signals, are receivedfrom various service providers at the central office (CO) 16. Thecentral office (CO) 16 converts the multimedia signals into opticalsignals at different wavelengths and multiplexes the various opticalmultimedia signals onto single fibers in the fiber optic network 18-23.The fiber optic network 18-23 distributes the optical signals to opticalnetwork termination (ONT) units 12, 14, which filter or demultiplex theoptical signals into their individual multimedia components, and convertthe filtered optical signals into electrical signals for use in the homeor office. An upgraded SCM ONT unit 14 may filter a received opticalsignal into its baseline (telephony/data) and SCM components, and anon-upgraded baseline ONT unit 12 may filter a received optical signalinto a baseline signal without being affected by the SCM component ofthe received signal. In addition, in a bidirectional system, both thenon-upgraded baseband ONT units 12 and the upgraded SCM ONT units 14 mayconvert baseline transmissions from the home (i.e., upstreamtelephony/data signals) into optical signals at a different opticalwavelength than the incoming (i.e., downstream) signals and transmit thesignals over the fiber optic network 18-23 to the CO 16.

-   -   A. Fiber Optic Network

The fiber optic network 18-23 shown in FIG. 1 is a point-to-multipointsingle fiber network that includes an outside plant 20, 21, a pluralityof passive remote splitters 23, and a plurality of distributionsplitters 18. The outside plant 20, 21 includes individual optic fibersor bundles of individual optical fibers with each individual fibercoupled between a route protection switch 46 in the central office 16and a passive remote splitter 23. The illustrated embodiment includestwo optic paths, a main path 20 and a redundant path 21, between thecentral office 16 and the passive remote splitters 23. Each routeprotection switch 46 in the central office 16 is, therefore, coupled toone passive remote splitter 23 via two individual optic fibers—one mainfiber and one redundant fiber.

Each individual fiber and its redundant pair in the illustrated outsideplant 20, 21 may provide service to thirty-two (32) homes. The passiveremote splitters 23 are eight-to-two (8:2) splitters that divide themain and redundant fibers 20, 21 into eight distribution fibers. Theoptical signals are transmitted for short distances over thedistribution fibers, without amplification, before termination at afour-to-one (4:1) distribution splitter 18 located in close proximity tofour ONT units. A distribution splitter 18 terminates a distributionfiber to four single drop fibers 19 that extend from the distributionsplitter 18 to a home or office and terminate at an ONT unit 12, 14. Thedistribution splitters 18, the fiber drops 19, and the ONT units 12, 14are added to the system 10 as service is required.

-   -   B. Central Office

The central office 16 shown in FIG. 1 includes a passivecross-connection unit 22, an optical video distribution sub-system 25,two baseline optical line termination (OLT) units 24, and a routeprotection control circuit 52. In operation, the central office 16interfaces the fiber optic network 18-23 with communication serviceproviders, such as CATV, DBS, and telephony/data services.

The baseline OLT units 24 each include a baseline splitting-blockingfilter combination 56, a downstream (DS) optical-electrical converter(OEC) 58, and an upstream optical-electrical converter 60. Thedownstream OEC 58 receives telephony/data signals from POT and datanetwork service providers, for example over a public telephone network,and converts the telephony/data signals into optical signals at aselected optical bandwidth for transmission to the ONT units 12, 14.Each baseline OLT unit 24 may provide optical telephony/data signals,without amplification, to a set number of ONT units 12, 14. For example,in the illustrated embodiment, each baseline OLT unit 24 can supplythirty-two (32) ONT units 12, 14. If more than thirty-two (32) ONT units12, 14 require baseline service, then additional baseline OLT units 24must be added at the CO 16. The illustrated CO 16 includes two baselineOLT units 24, and can thus supply sixty-four (64) ONT units 12, 14,without amplification.

The baseline splitting-blocking filter combination 56 in an OLT unit 24receives an optical downstream signal from the downstream OEC 58 andalso receives an upstream signal from the fiber optic network 18-23 viaa fiber connection 62 with the passive cross-connection unit 22. Thebaseline splitting-blocking filter combination 56 passes the downstreamtelephony/data signals to the fiber connection 62 with the passivecross-connection unit 22, and splits the upstream (US) telephony/datasignal received from the passive cross-connection unit 22. The isolatedupstream (US) telephony/data signals are converted into electricalsignals by the upstream OEC 60, and are transmitted to the serviceprovider. In a bi-directional system, the upstream (US) telephony/datasignals are transmitted at a different bandwidth and on the same opticfiber as the downstream (DS) telephony/data signals. Exemplary opticalbandwidths for the upstream and downstream telephony/data signals aredescribed below with reference to FIG. 2.

The optical video distribution sub-system 25 includes an SCM module 26,a high power optical amplifier 28, and an optical splitter 30. The SCMmodule 26 receives video signals, such as CATV or DBS signals, thatenter the CO 16 from the service provider head-end and/or satellite. TheSCM module 26 combines the video signals from the service providers intoone optical signal at a selected bandwidth. An exemplary opticalbandwidth for the SCM signal is described below with reference to FIG.2. The optical SCM signal from the SCM module 26 is then amplified bythe high power optical amplifier 28, and split into a plurality ofoptical SCM transmission signals by the optical splitter 30.

The passive cross-connection unit 22 includes a plurality of wavedivision multiplexers (WDMs) 54 and a plurality of optical routeprotection switches 46. The WDMs 54 each include two inputs—one inputcoupled to an optical SCM signal generated by the optical videodistribution sub-system 25, and one input coupled to the optical output62 from a baseline OLT unit 24. The WDMs 54 combine the downstreamtelephony/data signal from the baseline OLT unit 24 and the SCM signalfrom the optical video distribution sub-system 24 into one broadbandoptical signal.

The broadband optical signals generated by the WDMs 54 are coupled tothe fiber optic network 18-23 through the optical route protectionswitches 46. Each optical route protection switch 46 includes an inputthat receives a broadband optical signal from a WDM 54, a control inputfrom the route protection control circuit 52, and two optical fiberoutputs 48, 50. The two optical fiber outputs 48, 50 from an opticalroute protection switch 46 are coupled to a passive remote splitter 23through separate paths in the outside plant 20, 21. The route protectioncontrol circuit 52 monitors the optical continuity of the outside plant20, 21, and routes the broadband optical signal through either the mainor redundant path 20, 21 to adjust for any discontinuity. For example,the optical route protection switches 46 may be configured to connectthe broadband optical signals to the main path 48, 20 during normaloperation. If the route protection control circuit 52 detects adiscontinuity in the main path 20, then the route protection controlcircuit 52 may cause one or more of the optical route protectionswitches 46 to switch the broadband optical signals from one or moreindividual optic fibers in the main path 20 to the correspondingredundant fibers in the redundant path 50, 23 in order to preventinterruptions in service.

-   -   C. Optical Network Termination (ONT) Units

Two types of optical network termination (ONT) units are illustrated inFIG. 1: a SCM upgraded ONT unit 14, and a non-upgraded baseline ONT unit12. Both the SCM upgraded ONT units 14 and the non-upgraded baseline ONTunits 12 may be used to transmit and receive baseline telephony/datasignals via the fiber optic network 18-23. The SCM upgraded ONT units12, however, may also receive broadcast video signals, such as CATV orDBS signals, over the same optic fiber without affecting baselineservice to the non-upgraded ONT units 12.

A baseline ONT unit 12 includes a baseline splitting-blocking filtercombination 32, a downstream (DS) optical-electrical converter (OEC) 36,and an upstream optical-electrical converter (OEC) 34. The baselinesplitting-blocking filter combination 32 receives a broadband opticalsignal 32 from a fiber drop 19 and filters the signal 32 to isolatedownstream (DS) telephony/data signals 35, which fall within adesignated optical bandwidth. The downstream (DS) telephony/data signalsare coupled to the downstream (DS) optical-electrical converter (OEC)36, which converts the optical telephony/data signals into electricalsignals for use by equipment within the home or office. In addition, thebaseline splitting-blocking filter combination 32 also receives upstream(US) telephony/data signals 33 and passes the upstream (US) signals tothe fiber drop 19 for transmission to the central office 16 via thefiber optic network 18-23. The upstream (US) telephony/data signals aregenerated by equipment within the home or office, and converted tooptical signals by the upstream (US) OEC 34. As noted above, theupstream (US) telephony/data signals in a bi-directional system aretransmitted at a different bandwidth and on the same optic fiber as thedownstream (DS) telephony/data signals.

An upgraded SCM ONT unit 14 is similar to the baseline ONT unit 12, withthe inclusion of a SCM splitting-blocking filter combination 40 and aSCM optical-electrical converter (OEC) 42. The broadband optical signal38 from the fiber drop 19 is received by the SCM splitting-blockingfilter combination 40 which filters the broadband signal 38 to isolateoptical SCM signals 41 and to pass baseline telephony/data signals 44.The isolated SCM signals 41 are coupled to the SCM OEC 42, and theisolated baseline telephony/data signals 44 are coupled to the baselinesplitting-blocking filter combination 32. The SCM optical-electricalconverter 42 converts the optical SCM signals 41 into electrical signalsfor use by video equipment within the home or office. The baselinesplitting-blocking filter combination 32 filters the baselinetelephony/data signals 44 to isolate downstream (DS) telephony/datasignal 35 which are converted to electrical signals by the downstream(DS) OEC 36. In addition, both the baseline splitting-blocking filtercombination 32 and the SCM splitting-blocking filter combination 40 passupstream (US) telephony/data signals 33 for transmission to the centraloffice 16 via the optical network 18-23.

-   -   D. Optical Bandwidths

FIG. 2 is a graph illustrating exemplary bandwidths for thecommunication signals transmitted over the single fiber passive opticalnetwork 10 shown in FIG. 1. As illustrated, an upstream (US)telephony/data signal 102, a downstream (DS) telephony/data signal 104and a SCM signal 106 are each transmitted at different wavelengths overthe same optic fiber. The upstream (US) telephony/data signal 102 mayhave a bandwidth of about 1260-1360 nm, the downstream (DS)telephony/data signal 104 may have a bandwidth of about 1480-1500 nm,and the SCM signal 106 may have a bandwidth of about 1535-1565 nm. Alsoillustrated in FIG. 2 are the filter characteristics of the baseline andSCM splitting-blocking filters 32, 40, 56 in the ONTs 12, 14 and at thecentral office 16. The baseline splitting-blocking filters 32, 56include a band-pass filtering characteristic (between about 1460 and1525 nm) that isolates or passes downstream (DS) signals, and a low-lassfiltering characteristic (below about 1430 nm) that isolates or passesupstream (US) signals. The SCM splitting-blocking filter 40 includes aband-pass filtering characteristic (between about 1510 and 1580 nm) thatisolates SCM signals.

As noted above, the passive optical network 10 may be a bi-directionalsystem in which both upstream and downstream signals are transmitted, atdifferent wavelengths, on the same optic fiber. The downstream (DS)telephony/data signals 104 and the SCM signals 106 are multiplexed intobroadband signals that are transmitted from the central office 16 toboth the non-upgraded baseline ONT units 12 and the upgraded SCM ONTunits 14. The direction of the multiplexed SCM and DS signals 104, 106in the bi-directional system 10 is illustrated in FIG. 2 by the downwardpointing arrows. The upstream (US) telephony/data signals 102 aretransmitted from the ONT units 12, 14 to the central office 16 over thesame optic fibers as the multiplexed DS and SCM signals 104, 106. Thedirection of the US signals 102 in the bidirectional system 10 isillustrated in FIG. 2 by the upward pointing arrow.

II. Course Wavelength Division Multiplexing (CWDM) T-Band Upgrade

FIG. 3 is a block diagram of an exemplary single fiber passive opticalnetwork (PON) wavelength division multiplex overlay 200 that includes acourse wavelength division multiplexing (CWDM) T-Band upgrade. This CWDMT-Band upgraded PON 200 is similar to the SCM upgraded PON 10 describedabove with reference to FIGS. 1 and 2, except the CWDM T-Band upgrade200 also enables T-Band CWDM signals to be transmitted on the same opticfibers as the baseline and SCM signals without affecting subscribersthat have not upgraded to a T-Band upgraded optical network termination(ONT) unit 202. The CWDM T-Band upgrade may, for example, be added to abaseline system or the SCM upgraded PON 10 described above without anysubstantial effect to the existing services.

-   -   A. Central Office

The central office 16 in the CWDM T-Band upgraded PON 200 includes thepassive cross-connection unit 22, optical video distribution sub-system25, and baseline optical line termination (OLT) units 24, as describedabove with reference to FIG. 1. In addition, the central office 16 isupgraded to include a CWDM optical line termination (OLT) unit 210 andan additional WDM multiplexer 214. It should be understood, however,that in other embodiments the CWDM upgrade may be added to a baselinePON that does not include an SCM upgrade.

The CWDM OLT unit 210 in the CO 16 receives multimedia transmissionsfrom a service provider and multiplexes the multimedia signals into anoptical downstream T-Band signal 208. A T-Band signal may, for example,be used to provide an upgraded CWDM ONT unit 202 with a higher data ratelink than that available from a baseline service, or for other highbandwidth applications. In addition, several different multimediasignals may be simultaneously transmitted at different wavelengthswithin the multiplexed CWDM T-Band signal 208. For instance, a CWDMservice provider may offer one type of service, such as a video or dataservice, carried over one wavelength in the T-Band signal, and anothertype of service, such as voice, simultaneously carried over anotherwavelength.

The downstream T-Band signal 208 generated by the CWDM OLT 210 iscombined with a downstream (DS) baseline signal by the WDM multiplexer214 to generate a multiplexed CWDM/baseline output signal 216. Theoutput signal 216 from the WDM multiplexer 214 is then coupled as one ofthe inputs to a WDM multiplexer 54 in the cross-connection unit 22,which combines the multiplexed CWDM/baseline signal 216 with a SCMsignal from the optical video distribution sub-system 25 to generate thebroadband signal transmitted over the optical network 12-23. Withrespect to incoming signals from the ONTs 12, 14, 202, the additionalWDM multiplexer 214 also operates as a demultiplexer to separateupstream T-Band signals generated at a T-Band upgraded ONT unit 202 fromupstream (US) baseline signals. The isolated upstream (US) baselinesignals are coupled to the baseline OLT 24 and transmitted to thebaseline service provider as described above. The isolated upstream CWDMT-Band signals are coupled to the CWDM OLT 210, which separates theT-Band signal into its multimedia components, and transmits the signalsto the service provider.

-   -   B. T-Band Upgraded ONT Unit

The T-Band upgraded ONT unit 202 includes a CWDM ONT unit 204, a WDMmultiplexer (W1) 206, and either a baseline ONT unit 12 or a SCMupgraded ONT unit 14. In operation, the T-Band upgraded ONT unit 202 maysend and receive T-Band signals 208 over the fiber optic network 18-23using the CWDM ONT unit 204, and, depending on the additional servicespurchased, may also receive baseline telephony/data service and videoservice with an integral baseline or SCM upgraded ONT unit 12, 14.

The WDM multiplexer (W1) 206 in the T-Band upgraded ONT unit 202 iscoupled to a fiber drop 19 in the fiber optic network 18-23, andreceives incoming broadband optical signals that may include downstreambaseline, SCM, and downstream T-Band signal components. With respect tothe incoming signals, W1 206 operates as a demultiplexer to separate thedownstream T-Band signal components from the downstream baseline and SCMsignal components. The downstream T-Band signals are coupled to the CWDMONT unit 202, and the downstream baseline and SCM signals are coupled tothe integral baseline or SCM upgraded ONT unit 12, 14. The CWDM ONT unit202 splits the downstream T-Band signal 208, and demultiplexes the CWDMT-Band signal 208 into its multimedia components. In addition, upstreamCWDM T-Band signals 208 generated by the CWDM ONT unit 202 are combinedwith upstream (US) baseline signals by the WDM multiplexer (W1) 206, andare transmitted to the CO 16 via the fiber optic network 18-23.

-   -   C. Optical Bandwidths

FIG. 4 is a graph 300 illustrating exemplary bandwidths for thecommunication signals transmitted over the single fiber passive opticalnetwork 200 shown in FIG. 3. As illustrated, an upstream (US)telephony/data signal 102, a downstream (DS) telephony/data signal 104,a SCM signal 106, a downstream T-Band signal 302, and an upstream T-Bandsignal 304 are each transmitted at different wavelengths over the sameoptic fiber. As noted above, the upstream (US) telephony/data signal 102may have a bandwidth of about 1260-1360 nm, the downstream (DS)telephony/data signal 104 may have a bandwidth of about 1480-1500 nm,and the SCM signal 106 may have a bandwidth of about 1535-1565 nm. Inaddition, the downstream and upstream T-Band signals 302, 304 may fillthe available bandwidth (1360-1480 nm) between the US and DS baselinesignals 120, 104. The downstream T-Band signals 302 may have a bandwidthof about 1360-1430 nm, and the upstream T-Band signal 304 may have abandwidth of about 1430-1480 nm.

Also illustrated in FIG. 4 are the filter characteristics of thebaseline and SCM splitting-blocking filters 32, 40, 56 in the ONTs 12,14 and at the central office 16, as described above. In addition, thedirection (i.e., upstream or downstream) of the signals is illustratedin FIG. 4 by the direction of the arrows above the bandwidth for theparticular signal type. Upward-facing arrows represent upstream signals,and downward-facing arrows represent downstream signals.

III. Dense Wavelength Division Multiplexing (DWDM) L-Band Upgrade

FIG. 5 is a block diagram of an exemplary single fiber passive opticalnetwork wavelength division multiplex overlay 400 that includes a densewavelength division multiplexing (DWDM) L-Band upgrade. This DWDM L-Bandupgraded PON 400 is similar to the CWDM T-Band upgraded PON 200described above with reference to FIG. 3, except the DWDM L-Band upgrade400 also enables L-Band DWDM signals to be transmitted on the same opticfibers as the baseline, SCM, and T-Band signals without affectingsubscribers that have not upgraded to an L-Band upgraded ONT unit 402.

-   -   A. Central Office

The central office (CO) 16 in the DWDM L-Band upgraded PON 400 includesoptical video distribution sub-system 24, baseline OLT units 24, andCWDM OLT unit 210, as described above with reference to FIG. 3. Inaddition, the CO 16 is upgraded to include a DWDM OLT unit 410, and thecross-connection unit 22 is upgraded to include an additional WDMmultiplexer (W2) 414. It should be understood, however, that in otherembodiments the DWDM L-Band upgrade could be added to a baseline PONthat does not include a SCM or CWDM upgrade.

The DWDM OLT unit 410 in the CO 16 receives multimedia transmissionsfrom a service provider and multiplexes the multimedia signals into anoptical downstream L-Band signal 408. The DWDM multiplexing schemeemployed by the DWDM OLT unit 410 is similar to the CWDM multiplexingscheme of the CWDM OLT unit 210, as described above. An L-Band DWDMmultiplexer, however, combines multiple signals at a higher frequencyand less sensitive bandwidth than a T-Band CWDM multiplexer, and can,therefore, combine more signals into a lesser amount of bandwidth. Otheradvantages of L-Band transmission over T-Band transmission are generallyknown to those skilled in the art of passive optical networks.

The downstream L-Band signal 408 generated by the DWDM OLT 410 iscoupled to an input of the additional WDM multiplexer (W2) 414 in thecross-connection unit 22, which combines the L-Band signal 408 with abroadband signal generated by one of the other WDM multiplexers 54 inthe cross-connection unit 22. With respect to incoming signals from theONTs 12, 14, 202, 402, the additional WMD multiplexer (W2) 414 in thecross-connection unit 22 operates as a demultiplexer to separateupstream L-Band signals 408 generated as an L-Band upgraded ONT unit 402from other upstream signals. The isolated upstream L-Band signals 408are coupled to the DWDM OLT 410, which separates the L-Band signal intoits multimedia components, and transmits the signals to a serviceprovider.

-   -   B. L-Band Upgraded ONT unit

The L-Band upgraded ONT unit 402 includes a DWDM ONT unit 404, a WDMmultiplexer (W2) 406, and either a baseline ONT unit 12 or a SCMupgraded ONT unit 14. In operation, the L-Band upgraded ONT unit 402 maysend and receive L-Band signals 408 over the fiber optic network 18-23using the DWDM ONT unit 404, and, depending on the additional servicespurchased, may also receive baseline telephony/data service and videoservice with an integral baseline or SCM upgraded ONT unit 12, 14.

The WDM multiplexer (W2) 406 in the L-Band upgraded ONT unit 402 iscoupled to a fiber drop 19 in the fiber optic network 18-23, andreceives incoming broadband optical signals that may include downstreambaseline, SCM, downstream T-Band, and downstream L-Band components. Withrespect to the incoming signals, W2 406 operates as a demultiplexer toseparate the downstream L-Band signal components from other componentsof the incoming broadband signal. The downstream L-Band signals 408 arecoupled to the DWDM ONT unit 404, and the other signal components arecoupled to the integral baseline or SCM upgraded ONT unit 12, 14. TheDWDM ONT unit 404 splits the downstream L-Band signal 408, anddemultiplexes the L-Band signal 408 into its multimedia components. Inaddition, upstream DWDM L-Band signals 408 generated by the DWDM ONTunit 404 are combined with upstream (US) baseline signals by the WDMmultiplexer (W2) 406, and are transmitted to the CO 16 via the fiberoptic network 18-23.

-   -   C. Optical Bandwidths

FIG. 6 is a graph 500 illustrating exemplary bandwidths for thecommunication signals transmitted over the single fiber passive opticalnetwork 400 shown in FIG. 5. As illustrated, an upstream DWDM L-Bandsignal 502 and a downstream DWDM L-Band signal 504 are transmitted overa single optic fiber and at different wavelengths than baseline, SCM,and CWDM T-Band signals. As noted above, the upstream (US)telephony/data signal 102 may have a bandwidth of about 1260-1360 nm,the downstream (DS) telephony/data signal 104 may have a bandwidth ofabout 1480-1500 nm, the SCM signal 106 may have a bandwidth of about15351565 nm, the downstream T-Band signals 302 may have a bandwidth ofabout 1360-1430 nm, and the upstream T-Band signal 304 may have abandwidth of about 1430-1480 nm. In addition, the DWDM L-Band signals502, 504 may fill the available high-frequency bandwidth (15601600 nm)above the SCM signal 106. The upstream L-Band signal 502 may have abandwidth of about 1560-1580 nm, and the downstream L-Band signal 504may have a bandwidth of about 1580-1600 nm.

Also illustrated in FIG. 5 are the filter characteristics of thebaseline and SCM splitting-blocking filters 32, 40, 56 in the ONTs 12,14 and at the central office 16, as described above. In addition, thedirection (i.e., upstream or downstream) of the various signals isillustrated in FIG. 5 by the direction of the arrows above the bandwidthfor the particular signal type. Upward-facing arrows represent upstreamsignals, and downward-facing arrows represent downstream signals.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art.

1. An optical network termination unit, comprising: a first filteroperable to receive a broadband optical signal from a fiber drop, thebroadband optical signal including a downstream baseline optical signalcombined with a second downstream optical signal, the first filteroperable to isolate the second downstream optical signal from thebroadband optical signal; a second filter operable to isolate thedownstream baseline optical signal from a remaining portion of thebroadband optical signal after the second downstream optical signal hasbeen isolated from the broadband optical signal.
 2. The optical networktermination unit of claim 1, wherein the second downstream opticalsignal is a T-Band signal.
 3. The optical network termination unit ofclaim 2, further comprising: a wave division multiplexer that separatesthe T-Band signal from the broadband optical signal and demultiplexesthe T-Band signal.
 4. The optical network termination unit of claim 1,wherein the second downstream optical signal is an L-Band signal.
 5. Theoptical network termination unit of claim 4, further comprising: a wavedivision multiplexer that separates the L-Band signal from the broadbandoptical signal and demultiplexes the L-band signal.
 6. The opticalnetwork termination unit of claim 1, further comprising: a converterunit operable to receive an upstream baseline signal and convert theupstream baseline signal into an upstream baseline optical signal. 7.The optical network termination unit of claim 6, wherein: the upstreambaseline optical signal is within an optical bandwidth different from anoptical bandwidth of the downstream baseline optical signal and anoptical bandwidth of the second downstream optical signal; the upstreambaseline optical signal, the downstream baseline optical signal, and thesecond downstream optical signal are transmitted through the fiber drop.8. The optical network termination unit of claim 6, further comprising:a converter unit operable to receive a second upstream signal andconvert the second upstream signal into a second upstream opticalsignal, the second upstream optical signal being combined with theupstream baseline optical signal for transmission through the fiberdrop.
 9. The optical network termination unit of claim 8, wherein theconverter unit generates the second upstream optical signal in a T-bandsignal format.
 10. The optical network termination unit of claim 8,wherein the converter unit generates the second upstream optical signalin a L-band signal format.
 11. A method for upgrading a passive opticalnetwork, comprising: receiving a broadband optical signal from a fiberdrop, the broadband optical signal including a downstream baselineoptical signal combined with a second downstream optical signal;isolating the second downstream optical signal from the broadbandoptical signal; isolating the downstream baseline optical signal from aremaining portion of the broadband optical signal after the seconddownstream optical signal has been isolated from the broadband opticalsignal.
 12. The method of claim 11, further comprising: the seconddownstream optical signal is a T-Band signal; separating the T-Bandsignal from the broadband optical signal.
 13. The method of claim 12,further comprising: demultiplexing the T-Band signal.
 14. The method ofclaim 11, further comprising: the second downstream optical signal is anL-Band signal; separating the L-Band signal from the broadband opticalsignal.
 15. The method of claim 14, further comprising: demultiplexingthe L-Band signal.
 16. The method of claim 11, further comprising:passing an upstream signal through the fiber drop.
 17. A system forupgrading a passive optical network, comprising: means for receiving abroadband optical signal from a fiber drop, the broadband optical signalincluding a downstream baseline optical signal combined with a seconddownstream optical signal; means for isolating the second downstreamoptical signal from the broadband optical signal; means for isolatingthe downstream baseline optical signal from a remaining portion of thebroadband optical signal after the second downstream optical signal hasbeen isolated from the broadband optical signal.
 18. The system of claim17, further comprising: means for passing an upstream signal through thefiber drop on a separate bandwidth from the downstream baseline opticalsignal and the second downstream optical signal.
 19. The system of claim17, further comprising: means for separating a third downstream opticalsignal from the broadband optical signal, the third downstream opticalsignal being a T-band signal transmitted on a separate bandwidth fromthe downstream baseline optical signal and on a separate bandwidth fromthe second downstream optical signal.
 20. The system of claim 18,further comprising: means for separating a fourth downstream opticalsignal from the broadband optical signal, the fourth downstream opticalsignal being a L-band signal transmitted on a separate bandwidth fromthe downstream baseline optical signal, the second downstream opticalsignal, and the third downstream optical signal.